The present invention relates to printers, as well as to other types of electrophotographic image forming apparatuses, that provide visible images developed using toner or another powder, and to an exposure device, which is employed for an electronic image forming apparatus, that uses light to irradiate, and thus expose, a photosensitive member, which serves as an image carrier.
As described in patent document 1 and patent document 2, there are certain problems that are encountered with an electrophotographic image forming apparatus, such as a laser printer or an LED (Light-Emitting Diode) printer that uses colored toner, or another powder, to develop images, because during the development process the toner is dispersed internally and becomes attached to various operating parts of the apparatus. In addition to the problems caused by toner, like problems are encountered that are the result of the dispersal, like powder, of so-called lint produced from paper that is used as the print medium to which an image is transferred.
For an image forming apparatus, such as an LED printer, that prepares as a light source for exposure an LED array formed of a plurality of LEDs, and that to expose a photosensitive member, i.e., an image carrier, employs an exposure device that sequentially switches the LEDs on, the pitch of the LED array is generally almost equal to the pitch of the pixels (corresponding to the resolution) used to form an image. Thus, in order to reduce the size of the image forming apparatus, the exposure device is attached close to the photosensitive member, i.e., the image carrier. On the other hand, there is a considerably higher probability, in this case, that toner will be attached to the surface of a lens, the optical member of the exposure device, compared with the arrangement employed for a laser printer, wherein the exposure device is arranged at a specified distance from the image carrier. Therefore, for this type of image forming apparatus, problems occasioned by the dispersal and attachment of toner, paper lint and other dust-like particles to the optical member is more serious.
To resolve this problem, according to patent document 1, in which is described an embodiment for an LED printer, an image forming apparatus is disclosed wherein a cover, in which an opening is provided, is attached to the distal end of an exposure device; wherein, for the cover, a 1 mm or narrower space is set for a gap between the width of the opening and the width of a light beam and for a gap between the distal end of the exposure device and a photosensitive drum; and wherein a predetermined potential is applied to the distal end of the exposure device.
According to the description presented in Japanese Utility Model No. 2,544,078, the tiny gap, which is formed between the photosensitive drum and the exposure device, and an electrostatic force, which is used for charging toner, are employed to prevent toner, dispensed by a developing device, from entering the opening of an exposure device.
According to JP-A-10-048949, in order to prevent the inward dispersion of toner from the ends of a developing roller shaft, a technique is disclosed that provides for urethane foam sealing members to be formed along the faces of a support member and a case body that are opposite a photosensitive drum and along the portions of the faces of the support member and the case body that are opposite each other, and the sealing members are sandwiched by the photosensitive drum, the case body and the support member to provide a tight seal for a developing area.
According to the arrangement disclosed in Japanese Utility Model No. 2,544,078, a predetermined voltage is applied to the distal end of the cover, and by employing the electrostatic force to charge toner, the charged toner, when dispersed, reacts with and is repelled by the distal end, and is prevented from entering the opening (in the cover) of the exposure device. However, since the LED printer head is provided along almost the entire length (corresponding to the width of a printing sheet) of the barrel of the photosensitive drum, even though the gap between the cover and the barrel of the photosensitive drum is tiny, the total space between the cover, up to the distal end, and the photosensitive drum is quite large. Therefore, the entry of toner through the gap can not be completely prevented.
Furthermore, the technique disclosed in JP-A-10-048949 is basically intended to prevent the dispersion of toner supplied from the developing device. Even if the dispersion of toner from an area being developed can be prevented, by using a developing device for which the above described seal has been provided, a powder similar to talc (such as so-called paper lint) that is rubbed off a recording sheet will still be floating inside the image forming apparatus. This powder can be attached to the light output face of an exposure device, such as the LED, and the phenomenon can not be completely prevented that produces the deterioration of the image quality.